Systems and methods for RFID tag locating using constructive interference

ABSTRACT

A system and method for locating radio-frequency identification tags within a predetermined area. The method can incorporate sub-threshold superposition response mapping techniques, alone, or in combination with other methods for locating radio-frequency identification tags such as but not limited to time differential on arrival (TDOA), frequency domain phase difference on arrival (FD-PDOA), and radio signal strength indication (RSSI). The system can include a plurality of antennas dispersed in a predefined area; one or more radio-frequency identification tags; a radio-frequency transceiver in communication with said antennas; a phase modulator coupled to the ra-dio-frequency transceiver; and a system controller in communication with said transceiver and said phase modulator. Calibration techniques can be employed to map con-structive interference zones for improved accuracy.

RELATED APPLICATIONS

This application claims priority to International Application No.PCT/US2015/057206, entitled “Systems and Methods for RFID Tag LocatingUsing Constructive Interference,” filed Oct. 23, 2015, which in turnclaims priority to U.S. Provisional Application No. 62/067,736, entitled“System and Methods for RFID Tag Locating Using ConstructiveInterference,” filed Oct. 23, 2014. The disclosure of each of theseapplications is herein incorporated by reference in its entirety and forall purposes.

This application claims priority to U.S. Provisional Application No.62/067,736, entitled “System and Methods for RFID Tag Locating UsingConstructive Interference,” filed Oct. 23, 2014. The disclosure of U.S.Provisional Application No. 62/067,736 is herein incorporated byreference in its entirety and for all purposes.

TECHNICAL FIELD

This invention relates generally to the wireless communications field,and more specifically to new and useful systems and methods for usingconstructive interference such as use in radio frequency identificationtag (RFID) tag locating.

BACKGROUND

Being able to identify and track objects as they move throughoutbuildings or other indoor areas with high precision is important for awide variety of applications. Systems designed to track objects in thisway, often called real-time locating systems (RTLS), find use inmanufacturing, warehousing, retail inventory management, and medicine,to name a few areas. Unfortunately, current methods of tag locating usedby RTLS are frequently associated with cost and/or usability issues.Thus, there is the need in the wireless communications field to createsystems and methods for RFID tag locating. This invention provides suchnew and useful systems and methods.

SUMMARY

The present disclosure relates to a system for radio-frequencyidentification tag locating and associated methods.

In accordance with an aspect disclosed herein, there is set forth amethod for locating a radio-frequency identification tag, comprisinganalyzing first and second response signals received via a plurality ofantennas from the radio-frequency identification tag and determining aposition of the radio-frequency identification tag based upon saidanalyzing. In some embodiments, the determining can be accomplishedthrough triangulation.

In some embodiments of the method, the analyzing further comprisestransmitting an initial activation signal from one of the plurality ofantennas dispersed in a predefined area; and receiving a first responsesignal from the radio-frequency identification tag via the plurality ofantennas.

In some embodiments of the method, said transmitting comprisestransmitting via the selected antenna selected from among the pluralityof antennas.

In some embodiments, said transmitting comprises transmitting via theselected antenna selected from among the plurality of antennas beingdispersed in a predefined geographic area.

In some embodiments of the method, said analyzing comprises altering atransmission signal property of the initial activation signal,transmitting a secondary activation signal with the altered property;and receiving a second response signal from the radio frequencyidentification tag via the plurality of the antennas.

In some embodiments of the method, said receiving of the second responsesignal is conducted via the antennas.

In some embodiments, said transmitting the secondary activation signalcomprises broadcasting a plurality of sub-threshold radio-frequencyidentification signals to create a constructive interference patternwithin the predefined area.

In some embodiments, said transmitting comprises transmitting theplurality of sub-threshold radio-frequency identification signals viaseparate antennas.

In some embodiments of the method, said altering of the transmissionsignal property of the initial activation signal includes adjusting apower level of the secondary activation signal such that only a selectedradio-frequency identification tag located in an area of constructiveinterference will transmit the second response signal. The methodfurther comprises altering at least one of antenna power and a phase ofthe secondary activation signal to produce a second, but partiallyoverlapping, area of constructive interference.

In some embodiments of the method, said receiving includes: receiving aradio signal strength indication from a plurality of radio-frequencyidentification tag; and identifying only a selected tag located in afirst area of constructive interference due to a strength of thereceived radio signal strength indication.

In some embodiments of the method, said receiving includes: calculatinga read probability for a selected tag to respond when queried; and usingthe calculated read probability to locate the selected tag.

In some embodiments, the method further comprises mapping a responsepattern for the predefined area that compensates for environmental andstructural interference.

In some embodiments, said mapping includes traversing the predefinedarea with a robot with an attached radio-frequency identification tag,transmitting a plurality of calibration signals via the plurality ofantennas at respective power levels and phases, and comparing apredetermined location of the robot with a triangulated position of theattached radio-frequency identification tag.

In some embodiments, the mapping further comprises comparing thepredetermined location of the robot to a predicted constructiveinterference zone, comparing constructive interference data to predictedinterference data, and adjusting at least one of signal power or phaseof the secondary activation signal until the constructive interferencedata matches the predicted interference data.

In some embodiments, the method further comprises disposing acalibration radio-frequency identification tag on a person; determininga location of the person as the person traverses the predetermined area;comparing the location of the person to a predicted constructiveinterference zone; comparing constructive interference data to predictedinterference data; and adjusting at least one of signal power or phaseof the secondary activation signal until the constructive interferencedata matches the predicted interference data.

In some embodiments, the location of the person is determined using acamera.

In some embodiments, the camera can be any one of an RGB camera, amonochrome visible light camera, a 3-dimensional camera, an infraredcamera, and an ultra violet camera.

In some embodiments, the method further comprises determining a volumeoccupied by a person and an associated object.

In some embodiments, the volume occupied by a person and an associatedobject informs a targeted constructive interference pattern.

In some embodiments, the camera can identify the presence of theradio-frequency identification tag or an object with a predefinedvolume.

In some embodiments, the method disclosed is employed in combinationwith at least one of other tag locating techniques. In some embodiments,the other techniques can include at least one of time difference ofarrival, frequency domain phase difference on arrival, received signalstrength indication measurement, and read probability measurement.

In some embodiments, the method includes receiving environmental dataincluding at least one of air humidity, air temperature, anenvironmental noise level, and a signal indicating a presence of peopleor objects in the preselected area, and adjusting signal power or phaseof the secondary activation signal based on the environmental data togenerate a constructive interference pattern. Sensors can be used todetermine the background environmental radiation level.

In some embodiments, the method further comprises converting the secondresponse signal from the radio-frequency identification tag from ananalog signal into a digital signal in order to identify aradio-frequency identification tag number.

In some embodiments of the method, said altering includes changing oneor more of an antenna radio pattern, an antenna orientation, a signaltransmission power level, a frequency of the activation signal, a phaseof the activation signal, and a beam-width of the activation signal tomodify a constructive interference patterns.

In some embodiments, the method further comprises using historicalradio-frequency identification tag location data to further refine theconstructive interference pattern.

In some embodiments, the method further comprises calculating a velocityof a moving radio-frequency identification tag; predicting a newlocation of the moving radio-frequency identification tag based on thevelocity of the moving radio-frequency identification tag; and alteringthe phase of the secondary activation signal based on the new locationof the radio-frequency identification tag.

In accordance with an aspect disclosed herein, there is set forth asystem for locating a radio-frequency identification tag, comprising aplurality of antennas dispersed in a predefined area; one or moreradio-frequency identification tags dispersed within the predefinedarea; a radio-frequency transceiver in communication with said antennas;a phase modulator electrically coupled to the radio-frequencytransceiver; and a system controller in communication with saidtransceiver and said phase modulator.

In some embodiments, the system controller enables sub-thresholdsuperposition response mapping to calculate the location of theradio-frequency tags within the predefined area.

In some embodiments, the plurality of antennas can comprise any one ofthe following antenna types including a patch antenna, a reflectedantenna, a wire antenna, a bow-tie antenna, an aperture antenna, aloop-inductor antenna, and a fractal antenna.

In some embodiments, the plurality of antennas can comprise more thanone type of antenna. In some embodiments, the plurality of antennas areconnected directly to the radio-frequency transceiver. In someembodiments of the system, the plurality of antennas are connected tothe radio-frequency transceiver through one or more antenna splitters.

In some embodiments of the system, the plurality of antennas are capableof both transmission and reception of signals from the radio-frequencyidentification tags. In some other embodiments, the plurality ofantennas are capable only of transmission or reception of signals.

In some embodiments of the system, the radio-frequency transceiver iscapable of transmitting and receiving signals in a 900 megahertzfrequency band.

In some embodiments, the radio frequency transceiver is capable ofmodulating a power level of a transmission signal.

In some embodiments, the system controller can calculate a location of aradio-frequency identification tag from radio-frequency identificationresponse data.

In some embodiments, the system controller can store maps ofconstructive interference patterns of a predetermined location in astorage device.

In accordance with an aspect disclosed herein there is set forth amethod for mapping a response pattern for a predefined area compensatingfor environmental and structural interference, comprising traversing thepredefined area with a robot with an attached radio-frequency tag;transmitting a plurality of calibration signals at respective powerlevels and phases via a plurality of antennas disposed in the predefinedarea; and comparing a predetermined location of the robot with atriangulated position of the attached radio-frequency identificationtag.

In some embodiments, the method further comprises comparing thepredetermined location of the robot to a predicted constructiveinterference zone; comparing constructive interference data to predictedinterference data; and adjusting at least one of signal power or phaseof the secondary activation signal until the constructive interferencedata matches the predicted interference data.

In some embodiments, the method further comprises disposing acalibration radio-frequency identification tag on a person; determininga location of the person as the person traverses the predeterminedlocation; comparing the location of the person to a predictedconstructive interference zone; comparing constructive interference datato predicted interference data; and adjusting at least one of signalpower or phase of the secondary activation signal until the constructiveinterference data matches the predicted interference data.

In some embodiments, the method further comprises saving constructiveinterference data to a database.

In accordance with an aspect disclosed herein, there is set forth acomputer implemented method suitable for implementation on a processorcomprising analyzing first and second response signals via a pluralityof antennas from the radio-frequency identification tag; andtriangulating a position of the radio-frequency identification tag basedupon said analyzing, wherein said analyzing and triangulating areperformed by a processor.

In some methods, said analyzing further comprises transmitting aninitial activation signal from one of a plurality of antennas dispersedin a predefined area; receiving a first response signal from theradio-frequency identification tag via the plurality of the antennas.

In some embodiments of the method, said analyzing further includesaltering a transmission signal property of the initial activationsignal; transmitting a secondary activation signal with the alteredproperty; and receiving a secondary response signal from the radiofrequency identification tag via the plurality of the antennas.

In some embodiments of the method, said transmitting comprisestransmitting the plurality of sub-threshold radio-frequencyidentification signals to create a constructive interference patternwithin the predefined area.

In some embodiments, said transmitting comprises transmitting theplurality of sub-threshold radio-frequency identification signals viaseparate antennas.

In some embodiments, said altering includes adjusting a power level ofthe secondary activation signal such that only the selectedradio-frequency identification tag located in an area of constructiveinterference will transmit the second response signal; and altering atleast one of antenna power and a phase of secondary activation signal toproduce a second, but partially overlapping, area of constructiveinterference.

In some embodiments, the method further comprises mapping a responsepattern for the predefined area that compensates for environmental andstructural interference.

In some embodiments, said mapping includes traversing the predefinedarea with a robot with an attached radio-frequency tag; transmitting aplurality of calibration signals via the plurality of antennas atrespective power levels and phases; and comparing a predeterminedlocation of the robot with a triangulated position of the attachedradio-frequency identification tag.

In some embodiments, the method further comprises comparing thepredetermined location of the robot to a predicted constructiveinterference zone; comparing constructive interference data to predictedinterference data; and adjusting at least one of signal power or phaseuntil the constructive interference data matches the predictedinterference data.

In some embodiments, the method further comprises disposing acalibration radio-frequency identification tag on a person; determininga location of the person as the person traverses the predetermined area;comparing the location of the person to a predicted constructiveinterference zone; comparing constructive interference data to predictedinterference data; and adjusting at least one of signal power or phaseof the secondary activation signal until the constructive interferencedata matches the predicted interference data.

In some embodiments of the method, the location of the person isachieved using a camera.

In some embodiments of the methods, the camera can be any one of an RGBcamera, a monochrome visible light camera, a 3-dimensional camera, aninfrared camera, and an ultra violet camera.

One embodiment of the method, further comprises determining a volumeoccupied by a person and an associated object. In some embodiments, thevolume occupied by a person and an associated object informs a targetedconstructive interference pattern. In some embodiments of the method,the camera can identify the presence of the radio-frequencyidentification tag or an object with a predefined volume.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram view of a system of a preferred embodiment.

FIG. 2 is a diagram view of a prior-art RSSI locating technique.

FIG. 3A is an example view of constructive interference patternsgenerated by a system of a preferred embodiment.

FIG. 3B is another example view of constructive interference patterngenerated by a system of the preferred embodiment.

FIG. 4A is an example view of constructive interference patternsgenerated by a system of a preferred embodiment.

FIG. 4B is another example view of constructive interference patternsgenerated by a system of the preferred embodiment.

FIG. 5 is a diagram view of a system of a preferred embodiment.

FIG. 6 is an example view of constructive interference patternsgenerated by a system of a preferred embodiment.

FIG. 7 is a chart view of a method of a preferred embodiment.

It should be noted that the figures are not drawn to scale and thatelements of similar structures or functions are generally represented bylike reference numerals for illustrative purposes throughout thefigures. It also should be noted that the figures are only intended tofacilitate the description of the preferred embodiments. The figures donot illustrate every aspect of the described embodiments and do notlimit the scope of the present disclosure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments of the inventionis not intended to limit the invention to these preferred embodiments,but rather to enable any person skilled in the art to make and use thisinvention.

1. RFID Tag Locating System

As shown in FIG. 1, a radio-frequency identification (RFID) tag locatingsystem 100 includes a plurality of antennas 110, an RFID transceiver120, a phase modulator 130, and a system controller 140. The system 100may additionally include one or more reference RFID tags 150.

The system 100 functions to locate RFID tags within a three-dimensionalvolume of interest (or a two-dimensional plane of interest). The system100 preferably determines tag location across time in order to trackchanges in tag location and/or tag movement. The system 100 ispreferably designed and used to locate ultra-high frequency (UHF)passive RFID tags, but may additionally or alternatively be designed andused to locate passive RFID tags operating on any frequency spectrum.Additionally or alternatively, the system 100 may also be used withactive RFID tags or any other suitable devices capable of respondingselectively based on received RF signal power.

Traditional RFID tag locating systems use one of several methods for taglocation, including time difference of arrival (TDOA), phase differenceof arrival (PDOA), and received signal strength indication (RSSI)measurement. All three of these methods can locate tags usingtrilateration.

In the case of TDOA, a signal is sent to an RFID tag from one of threeantennas. The tag receives the signal and transmits a signal inresponse. The response signal is then received at all three of theantennas at different times. The time between original signaltransmission and reception of the response signal at each antenna can beused to determine the distance from the tag to each antenna, which canthen be used to locate the RFID tag (relative to the antennas) usingtrilateration. The TDOA method is not typically used for UHF RFID tagssimply because typical time differences are very small (and bandwidthavailable is narrow).

There are several types of PDOA, including frequency domain PDOA(FD-PDOA). In FD-PDOA, a signal is sent to a tag from one of threeantennas at a first frequency; the tag responds with a first responsesignal. Then the same antenna sends a signal at a second frequency(preferably close to the first frequency), and the tag responds with asecond response signal. The phase difference between the first responsesignal and the second response signal (as measured at the first antenna)can give a distance from the tag to the first antenna. This process canbe repeated for the other two antennas, producing three distances, whichcan be used to locate the tag using trilateration.

In the case of RSSI measurement, as shown in FIG. 2, a signal is sent toan RFID tag from one (or more) of three antennas. The tag receives thesignal and transmits a signal in response. The response signal is thenreceived at all three of the antennas, each recording a differentreceived signal strength (e.g., RSSI). The RSSI is used to estimatedistance from each antenna, which can then be used to locate the tagrelative to the antennas using trilateration. Since RSSI does nottypically correspond well to distance, this method may suffer fromaccuracy issues.

Another method for locating RFID tags is known as read probabilitymeasurement, described in U.S. Provisional Patent Application No.61/928,303, which is incorporated in its entirety by this reference. Tobriefly summarize, read probability measurement takes advantage of RFIDtag power-on thresholds (that is, the minimum amount of power a passiveRFID tag must receive in order to transmit a readable response signal).The antennas modulate transmission power and record whether the tagresponds or not at each transmission power. A number of thesetransmissions are used together to calculate a read probability (theprobability that a tag will be read versus transmission power). Bycomparing this to an estimate or analysis of how transmission signalpower changes with distance (and potentially direction) for eachtransmission power, a distance from each antenna can be determined, andtrilateration can be performed.

The system 100 preferably locates RFID tags using a method henceforthreferred to as sub-threshold superposition response mapping (STSRM).This technique may be used independently of the method of RFID taglocating described previously, but may additionally or alternatively beused in conjunction with those methods.

As with previously described methods, STSRM (described in more detail inthe description of the method 200) also involves the use of multipleantennas; however, the locating process is very different frompreviously described methods. In STSRM, multiple antennas transmit asignal at the same time. The signals transmitted by the antennasinterfere with each other, creating areas of constructive interferenceand areas of destructive interference. Based on the relative location ofantennas, signal properties of the signal emitted by each antenna (e.g.,phase, polarization, beam width, etc.), and the environment (e.g.,obstacles in between antennas and tags), the interference patterngenerated by signals can be predicted. The power of each antenna can beadjusted such that only areas of strong constructive interference(preferably a sparse pattern) have enough power to activate RFID tags;in other words, the individual signals are sub-activation-threshold inareas of interest. If an RFID tag is activated, it must then lie in oneof these areas of constructive interference. After an RFID tag islocated within a constructive interference area, the constructiveinterference pattern is then changed (by altering antenna power andphase) to produce a different, but partially overlapping, set ofconstructive interference points. This process of altering patterns mayproceed until the RFID tag has been confined to a single location.

In other embodiments, instead of using power level for threshold foractivation of tags inside the target zone, received signal strengthindication (RSSI) measurement can be used. Outside the constructiveinterference zone, there will be a marked decrease in RSSI. The variedRSSI signal levels can produce a steep gradient for tags located insideand outside the zone. This difference in RSSI can be used to eliminatetags located outside the target zone. For tags that are located on theboundary of a constructive interference zone, other techniques can beutilized to determine whether the tag is located inside or outside thezone. Examples of the constructive interference patterns produced duringSTSRM are as shown in FIGS. 3A and 3B. FIGS. 3A and 3B include fieldcontour plots, where field strength above a threshold is displayed asblack and field strength below the threshold is displayed as white. Inthese examples, the plot threshold is chosen to be an RFID tagthreshold; that is, RFID tags will only have enough power to respond totransmitted signals if they are in black areas. The examples shown inFIG. 3A and FIG. 3B are identical except for transmission power; P2<P1.Because P2 is lower, fewer points in the area of interest aresuper-threshold (and thus potential locating resolution is higher). Moreexamples are as shown in FIGS. 4A and 4B; in these examples, thetransmission power is the same between FIGS. 4A and 4B, but the relativephase of antennas is different. Note that all of these examples assume auniform transmission media (e.g., air) and no reflections; these factorsare often important in determining real-world constructive interferencepatterns.

The locating process as described above discusses localization usingonly STSRM, but the technique may additionally or alternatively be usedwith other techniques to narrow the search field (i.e., how manypatterns must be tested) or for other purposes. For example, readprobability measurement may place an RFID tag within a 1×1 meter areawith 95% accuracy, and STSRM could be used to further narrow downlocation within this area.

The nature of constructive interference is that with multiple signalstraveling in different directions, effects can be highly localized (onorder of signal wavelength). Further, altering of phase can displaceinterference peaks by magnitudes substantially smaller than wavelength,meaning that STSRM is capable of achieving very high accuracy inlocating RFID tags.

The system 100 preferably enables the use of STSRM techniques to locateRFID tags; additionally or alternatively, the system. 100 may enable theuse of other tag locating techniques in combination with orcomplementary to STSRM techniques.

The antennas 110 function enable the system 100 to transmit signals toRFID tags and receive signals from the RFID tags. The antennas 110convert conducted electric power into RF waves and/or vice versa,enabling the transmission and/or reception of RF communication. Theantennas 110 are preferably made out of a conductive material (e.g.metal). The antennas 110 may additionally or alternatively includedielectric materials to modify the properties of the antennas 110 or toprovide mechanical support.

The antennas 110 may be of a variety of antenna types; for example,patch antennas (including rectangular and planar inverted F), reflectorantennas, wire antennas (including dipole antennas), bow-tie antennas,aperture antennas, loop-inductor antennas, and fractal antennas. Theplurality of antennas 110 can additionally include one or more type ofantennas, and the types of antennas can include any suitable variations.

The antenna 110 structure may be static or dynamic (e.g. a wire antennathat includes multiple sections that may be electrically connected orisolated depending on the state of the antenna).

Antennas 110 may have isotropic or anisotropic radiation patterns (i.e.,the antennas may be directional). If antennas 110 are directional, theirradiation pattern may be dynamically alterable; for example, an antenna110 substantially emitting radiation in one direction may be rotated soas to change the direction of radiation.

The plurality of antennas 110 are preferably connected directly to RFIDtransceivers 120 with conductive wires, but may additionally oralternatively be connected to transceivers through any suitable method.The antennas 110 may be connected directly to RFID transceivers 120, ormay be connected RFID transceivers 120 through one or more antennasplitters.

The system 100 preferably includes at least three antennas 110, so as tobe able to perform trilateration, but the system may additionallyinclude any suitable number of antennas. In one implementation of thesystem 100, the system 100 includes a rectangular grid of antennas 110.Other embodiments can selectively assign antennas to various roles. Inone embodiment a fixed number of antennas can be tasked with targeting aparticular zone, while other antennas can be assigned to reducingsecondary effects interference from other power zones which can occursome distance away from the targeted zone.

The antennas 110 of the system 100 are preferably used both fortransmission of signals to and reception of signals from RFID tags, butmay additionally or alternatively antennas may be used only fortransmission or only for reception.

Antennas 110 are preferably located as to provide coverage for aparticular indoor area. For example, antennas 110 might be oriented in arectangle on the ceiling of a store in order to locate RFID tagscontained within the rectangle. In this particular implementation, ofthe two solutions produced by trilateration, only one would be valid(the assumption being that no RFID tags are present above the ceiling).

The RFID transceiver 120 functions to produce signals for transmissionby the antennas 110, as well as to analyze signals received by theantennas 110 from RFID tags. In one embodiment, the RFID transceiverpreferably includes an RF transmitter capable of sending signals in the900 MHz band and an RF receiver capable of receiving signals in the 900MHz band, but may additionally or alternatively be any suitabletransceiver capable of communicating with RFID tags. The 900 MHz bandsupports 902-928 MHz in North America. Alternatively the transmitter canoperate in the 800 MHz band. The 800 MHz band supports 865-968 MHz inEurope. Alternatively, the transceiver can operate in the industrial,scientific and medical (ISM) radio band from 2.4-2.485 (Bluetooth Band),2.4 gigahertz (12 cm) UHF and 5 gigahertz (6 cm) SHF ISM radio bands,3.1-10 GHz (microwave band), and other UHF RFID tag emitter bands in useor later developed.

The RFID transceiver 120 is preferably coupled directly to the antennas110, but may additionally be coupled to the antennas 110 through anantenna splitter or through any other components.

The RFID transceiver 120 is preferably controlled by the systemcontroller 140, but may additionally or alternatively be controlled byany other component of the system 100. The RFID transceiver 120 ispreferably capable of modulating power to the antennas 110, additionallyor alternatively, power modulation may be accomplished by a deviceexternal to the RFID transceiver 120 (e.g., an active splitter).

The phase modulator 130 functions to change the phase of the signaloutput by one or more antennas 110. Changing the phase of any one of theantennas 110 has the effect of changing the far-field interferencepattern (and thus the areas that RFID tags may be activated in). Thephase modulator 130 is preferably part of the RFID transceiver 120, butmay additionally or alternatively be a component independent of the RFIDtransceiver 120.

If the phase modulator 130 is part of the RFID transceiver 120 and eachantenna 110 (or antenna array) is connected to the RFID transceiver 120individually (as shown in FIG. 1), the phase modulator 130 preferablychanges phase simply by modifying the digital signal intended for aparticular antenna. For example, the carrier wave of an RF signaltransmitted by an antenna 110 might have the form of cos[ωt+ϕ], where ϕrepresents an alterable phase shift. The phase modulator 130 may simplyadjust the value of ϕ to provide the signal with a particular phase.

If the phase modulator 130 is part of or after an antenna splitter, asshown in FIG. 5, or otherwise operates on the analog signals intendedfor the antennas 110 (as opposed to the previous example, where thephase modulator 130 operates in the digital domain), the phase modulator130 may consist of variable delay circuits connected to the antennas110. Additionally or alternatively, the phase modulator 130 may compriseany digital or analog circuit or component capable of altering the phaseof the transmitted signals of one or more antennas 110.

The system controller 140 functions to control the output of the RFIDtransceiver 120 and the phase modulator 130, as well as to process thesignals received by the RFID transceiver 120. The system controller 140includes a microprocessor; the system controller 140 may be integratedwith the RFID transceiver 120 and phase modulator 130, but mayadditionally or alternatively be separate of one or both of the RFIDtransceiver and phase modulator 130.

The system controller 140 enables the system 100 to transform RFIDresponse data into a location for an RFID tag. The system controller 140preferably accomplishes this transformation by using a mapping ofconstructive interference patterns to physical locations to estimate thecoordinates at which signal power rises above some activation threshold.This process is described in more detail in the sections on the method200.

The system controller 140 preferably includes a processor and storagefor the above-mentioned maps, but may additionally or alternativelystore map data and configuration data in any suitable location (e.g.,cloud-based servers).

The system controller 140 preferably performs this transformation usingstored maps. The system controller 140 may additionally or alternativelygenerate maps in real-time. These maps preferably allow the systemcontroller to determine super-threshold areas of constructiveinterference based on transmission variables; for example, the locationof antennas 110, the angle of orientation of antennas 110, the radiationpattern of antennas 110, the phase, frequency, polarization, and powerof signals transmitted by the RFID transceiver 120 (via the antennas110), or any other applicable data. The maps may additionally oralternatively vary based on environmental variables, for example, thenumber of people within the area of interest.

Constructive interference patterns may be strongly dependent onenvironment. For example, a change in positioning of shelves in a storemight cause larges changes in the constructive interference patterngenerated given a certain set of transmission parameters. For thisreason, it may be helpful for the system controller 140 to have acalibration reference; for example, data defining how constructiveinterference patterns for a particular area.

The calibration references may be static; for instance, the calibrationreferences may be formed by a robot with an RFID tag traversing an area;the robot maps out the area while the system 100 outputs one or moreconstructive interference patterns. The system 100 outputs constructiveinterference patterns by transmitting a signal from one or more antennas110 at particular transmission powers and phases. The robot map may be,through time synchronization, matched up to points of RFID tagactivation, this data set is then compared to constructive interferencedata predicted by the system controller 140. The system controller 140may then adjust transmission variables (e.g., by adjusting transmissionvariable inputs to a prediction engine until prediction matches reality,or by adjusting actual transmission variable inputs until the robotoutput matches predictions). Static calibration processes may beperformed in real-time with data gathering (e.g., as the robot movesaround) or at a later time (using previously corrected data).

Similarly, a robot may be used to map read probabilities for variouslocations within an area. For example, a robot may be used to map out anarea while the system 100 outputs signals from one antenna (or seriallyfrom multiple antennas). The robot map, through time synchronization,may be matched up to points of RFID tag activation; this data may thenbe used to calculate read probabilities as a function of position in thearea. As in the previous process, the system controller 140 may adjusttransmission variables to match predictions to reality or vice versa.Read probability calibration processes may be performed in real-timewith data gathering (e.g., as the robot moves around) or at a later time(using previously corrected data).

The calibration references may additionally or alternatively be dynamic.In one example, the system 100 includes RFID reference tags 150 placedin known locations. These may be used to calibrate or recalibrate thesystem controller 140 mapping at any time. This allows the system 100 tobe recalibrated easily when environmental factors (e.g., positioning ofRF-signal-affecting objects, etc.) change. The system 100 preferablycalibrates with references by predicting patterns that would activateparticular reference tags, testing those patterns, and refining thepatterns based on response or non-response.

Calibration may additionally or alternatively be performed with the aidof non-STSRM techniques. For example, persons in a particular area maycarry RFID tags that identify them. If the position of the persons canbe located with precision (e.g., by a camera, or by another method, suchas detecting wireless transmissions from their cellphone), the RFID tagsthey carry could be used to calibrate the system 100. Cameras or otherlocating methods may additionally or alternatively be used at any pointin order to refine or calibrate STSRM location data or the system 100.

For example, a camera (e.g., RGB camera, monochrome visible lightcamera, 3D camera, depth camera, infrared camera, or an ultra violetsensor etc.) could be used to recognize a person (either generically asa person, or as a particular person, using face recognition software,gait analysis, or another suitable technique). The camera mayadditionally or alternatively be used to calculate the volume occupiedby the person and associated objects (e.g., a shopping cart). Thelocation and volume occupied by the person and/or associated objectscould be used to inform a particular constructive interference pattern;for example, to query RFID tags of objects contained within the person'sbag or shopping cart. This could be used to determine particular items aperson is carrying. Additionally or alternatively, location information,recognition data, visual data, or any other suitable camera data may beused in combination with STSRM data in any suitable manner in order toprovide further information about the presence of RFID tags (or otherobjects) within a particular volume.

The system controller 140 may additionally or alternatively use theantennas 110 to perform calibration; for example, the system controller140 may transmit a signal at a first antenna 110 and receive it at asecond antenna 110. Because the relative locations of the antennas 110are preferably known, the signal can be used to determine delay or phaseshift due to environmental factors in the signal path. This informationcan be used to refine constructive interference pattern maps.

In addition to controlling the calibration process, the systemcontroller 140 preferably controls the transmissions used for RFID taglocation. The system controller 140 preferably adjusts phase andtransmission power to locate RFID tags in a small number of iterations(e.g., by optimizing for a minimum number of iterations given roughknowledge about the position of a tag). For example, the systemcontroller 140 may know from a previous search that a tag is located ina particular area. If analysis of historical data suggests that the tagis likely to be in the same area, the system controller 140 may attemptto isolate the search to this area before trying other areas. The systemcontroller 140 storage may analyze historical data related to taglocation in a number of ways. Historical data preferably includeshistorical environmental data, historical absolute location data (e.g.,the tag's location in coordinate space), historical relative locationdata (e.g., the tag's location relative to other tags or otherreferences), behavioral data (e.g., the tag is likely to be in themiddle of the area during the afternoon, but near the left edge duringthe evening), or any other suitable data.

The system controller 140 preferably alters phase and transmission powerof antennas 110 by controlling RF transceivers 120 and phase modulators130, but may additionally or alternatively alter antenna phase andtransmission power in any suitable manner.

The system controller 140 may locate RFID tags using only the STSRMmethod, but may additionally or alternatively locate RFID tags using acombination of methods; for instance, RSSI may be used to roughly locateRFID tags, and then STSRM may be used to locate RFID tags with higherresolution. If the system 100 performs multiple methods of tag locating,all methods are preferably directed by the system controller 140, butthe system controller 140 may alternatively direct only a subset oflocating methods.

The reference RFID tags 150 function to provide a calibration referenceto the system 100. The reference RFID tags are preferably substantiallysimilar to the RFID tags located by the system 100, but may additionallyor alternatively any suitable type of RFID tag. The RFID tags preferablyhave a known tag identifier (i.e., the signal transmitted by the tagwhen interrogated) and a known position. Thus, when reference RFID tags150 transmit, the system controller 140 can infer that the activationsignal was above-threshold at the location of transmitting referenceRFID tags.

Reference RFID tags are preferably associated with a location that isstatic relative to the antennas 110, but may additionally oralternatively be associated with a location in a different coordinatespace. For example, reference RFID tags may be located with GPScoordinates, or with some particular object (e.g., a moveable cart maycontain a reference RFID tag so that positions may be determinedrelative to that cart).

Sub-threshold superposition response mapping (STSRM) techniques are notlimited to the use of UHF radio frequency radiation. STRM techniques canbe applied using ultrasound radiation. Ultrasound devices operate withfrequencies from 20 kHz up to several gigahertz. Sound vibration canform constructive interference patterns similar to ultrahigh frequencyradiation and STSRM techniques can be applied for the selectivetransmission of sound waves.

In addition to using constructive interference mapping techniques forthe locating of RFID tags, these methods could be used for othertargeted transmission and receipt of energy. Such applications include,the targeted transmission of radiation resulting in constructiveinterference zones for the targeted transmission of energy for specificareas. This could be used for selected areas for transmitting radiationfor wireless, remote recharging of portable electronic devices. In thisway, concentration of the radiation to selective areas would reduce theharmful effects of radiation on humans with isometric radiation.

Other uses included the selected targeting of areas for concentratedbandwidth distribution. In this way some areas would have higherbandwidth capabilities in these constructive interference zones thanoutside the constructive interference zones. In zones outside the targetzone, the data rate would be significantly reduced.

As shown in FIG. 7, a method 200 for sub-threshold superpositionresponse mapping (STSRM) preferably includes transmitting a plurality ofsub-threshold RFID activation signals from separate antennas S210,receiving a response signal from an RFID tag S220, altering transmissionsignal properties S230, receiving an additional response signal from theRFID tag S240, and calculating the RFID tag position S250. The method200 may additionally include calibrating interference mapping S260.

The method 200 functions to locate RFID tags within a specific volume(bounded by antenna range). The method 200 preferably results in a moreaccurate location estimate than from typical methods (e.g., TDOA, PDOA,etc.).

Step S210 includes transmitting a plurality of sub-threshold RFIDactivation signals from separate antennas. Step S210 functions to createa constructive interference pattern within an area defined by thetransmitting antenna range. The constructive interference pattern is afunction of antenna and signal properties including antenna radiationpattern, antenna orientation, antenna type, transmission power,frequency, phase, beam-width, and other factors.

The locations of the antennas are preferably known relative to eachother. Antennas may additionally or alternatively be referenced to anycoordinate frame of reference.

The transmission power and relative phase of activation signals arepreferably set based on an estimated constructive interference pattern,but may additionally or alternatively be based on any suitableinstructions or data. The transmission power and relative phase ofactivation signals are preferably set such that only a small subset ofthe area covered by antenna range results in super-threshold signalpower; that is, most of the area covered by antenna range does not haveenough constructive interference to, activate an RFID tag.

The particular power and phase settings chosen for each signal arepreferably informed by historical data; that is, the interferencepattern generated by Step S210 is preferably intended to activate tagsin a particular subset of in-range area where the tags are assumed tobe. Additionally or alternatively, the power and phase settings chosenby Step S210 may result from explicit settings (e.g., the firstactivation signals always have a relative phase of zero and atransmission power of 100 dBm), other data (e.g., data from otherlocating methods), or any other suitable instructions.

Step S210 may additionally or alternatively include receivingenvironmental data (e.g., humidity, presence of people or objects,temperature, environmental RF noise, etc.) or previous mappinginformation (e.g., a mapping of particular transmission settings to aconstructive interference pattern). This data may be used to inform thetransmission settings in order to more accurately generate particularconstructive interference patterns. Previous mapping information orother calibration information preferably results from Step S260, but mayadditionally or alternatively come from any suitable source.

Step S220 includes receiving a response signal from an RFID tag. StepS220 functions to provide data that can be used to generate informationabout the RFID tag's location. Based on the transmission settings ofStep S210 and the predicted mapping of signal strength (taking intoaccount constructive interference), the location of the RFID tag may beconfined to a set of points (or small areas) of constructiveinterference. Note that Step 210 may need to be iterated multiple timesat different transmission settings before receiving a response signalfrom a particular RFID tag.

Step S220 preferably includes receiving an analog signal over one ormore antennas; these antennas are preferably the same antennas used totransmit signal in Step S210, but may additionally or alternatively beany suitable antennas. This analog signal is preferably converted to adigital signal and analyzed to provide the locating system with the RFIDtag ID. Additionally or alternatively, if the tag identifier is notimportant to a particular application, the signal may not be converted(e.g., an application that only cares about locating any tag, not aspecific tag).

Step S230 includes altering transmission signal properties. Step S230functions to change the constructive interference pattern used to enableRFID tag responses. Step S230 may occur after Step S210 (if a desiredtag is not located) or after Step S220 (to refine the location of aparticular tag).

Step S230 preferably includes altering one or more of antenna radiationpattern, antenna orientation, signal transmission power, frequency,phase, and beam-width in order to alter constructive interferencepatterns.

The alterations made by Step S230 preferably are informed by existingdata or estimates pertaining to an RFID tag's location; additionally oralternatively, alterations may be made according to a static instructionset or in any other suitable manner. For example, if analysis of datafrom Step S220 identifies an RFID tag as occupying a location in thefirst quadrant of a square area (i.e., x>0 and y>0) or in the thirdquadrant (x<0, y<0), and historical data suggests that the RFID tag ismuch more likely to be in the first quadrant, the alterations made byStep S230 may produce an interference pattern more likely to providelocation information on a tag located in the first quadrant.

As a specific example of data pertaining to RFID tag location, thealterations made by Step S230 are preferably informed by the results ofprevious alterations. For example, as shown in FIG. 6, a first patternmay be generated by Step S210, resulting in tag detection in Step S220.Step S230 alters the transmission signal to produce a second pattern,which results in no detection. Assuming that the tag did not movesignificantly between the generation of pattern 1 and pattern 2, the tagmust be located in the area found by subtracting pattern 2 from pattern1. In this example, Step S230 might then be run again, with the thirdpattern calculated to give more information about where the tag might belocated within the area defined by the removal of pattern 2 area frompattern 1. While this example includes a detection and a non-detection,the same principles apply to two detections in a row. For example, if anRFID tag were detected in both pattern 1 and pattern 2, the RFID tagwould be located within the intersection of pattern 1 and pattern 2(again assuming no substantial movement between responses).

Preferably, tags read by the method 200 do not move significantly whilebeing located; but if it is expected that tags will move significantlywhile being located, the method 200 may include detecting tag velocityand adjusting locating techniques appropriately (e.g., predicting wherea tag will be based on previously measured velocity and attempting tolocate it at the predicted location). Tag velocity may be detected inany number of ways, including by the steps previously mentioned.Altering antenna phase only slightly has an effect of essentiallyshifting the constructive interference pattern without substantiallyaltering it; by shifting constructive interference patterns slightly tagvelocity can be determined even if tag location is not definitely known.For example, if the method 200 confines tag location to a first set ofpoints defined by a constructive interference pattern, generates ashifted pattern and detects the tag in a second set of points defined bya second constructive interference pattern, the average velocity of thetag between the generation of those two patterns falls into a bound setof solutions. By performing additional pattern generations and/or byincluding some assumptions (e.g., maximum velocity the tag can move at,direction of velocity, etc.) the tag velocity can be determined.

Step S240 includes receiving an additional response signal from the RFIDtag. Step S240 is preferably substantially similar to Step S220. Theresults of the second response signal are preferably used in determiningRFID tag position; the results may additionally be used to direct StepS230 (e.g., by identifying an area of interest to search in).

Steps S230 and Steps 240 are preferably iterated until RFID tag locationhas been suitably confined. In some cases, Steps S230 and S240 may beiterated a set number of times; for instance, there may be a set ofconstructive interference patterns that can, to a desired resolution,locate any tag within an area of interest (regardless of tag locationwithin the area of interest) and Steps S230 and S240 may be iterateduntil this set has been completed. Additionally or alternatively, StepsS230 and Steps S240 are iterated along with an intermediate iteration ofStep S250; for example, after each iteration of Step S230 and Step S240,Step S250 uses the results to further confine tag location and to directparameters of the next iteration of Step S230, the iteration cyclecontinuing until Step S250 has suitably determined tag location (e.g.,by reducing possible tag location area to an area below some thresholdarea).

Step S250 includes calculating the RFID tag position. Step S250functions to determine or estimate where RFID tags are located based onresponses to particular interference patterns. Step S250 is preferablyiterated along with steps S230 and S240, but may additionally oralternatively be performed only after several iterations of Steps S230and S240 or at any suitable time.

Step S250 preferably calculates RFID tag position by correlating RFIDtag response or non-response to locations defined by constructiveinterference patterns. Step S250 preferably produces RFID tag positiondata from RFID tag response data and transmission parameter sets (e.g.,whether a tag responded or not for a particular transmission parameterset) by generating a transmission power field estimate (or otherdistribution correlated to RFID response rates) based on thetransmission parameter set.

The mapping between transmission parameter sets and transmission powerfields is preferably set by Step S260, but may additionally oralternatively be set in any suitable manner. As described in Step S260,the mapping may vary solely on transmission power and phase (i.e., allother transmission parameters, including antenna location, andenvironmental variables are considered static) or the mapping may varybased on additional variables. For example, the mapping algorithm mightalso vary based on the number of people known to be in a particular area(changing the permittivity of the area, and thus the interferencepattern) or based on antenna direction, if antenna direction isvariable.

Step S250 may additionally or alternatively include calculating RFID tagposition based on a combination of multiple locating methods (e.g., bylocating an RFID tag to a particular area using a read probabilitymethod and then locating the tag within that area using STSRM).

Step S260 includes calibrating interference mapping. Step S260 functionsto increase the accuracy of the mapping between antenna fields(specifically, super-threshold and sub-threshold areas) and location(relative to antennas or otherwise).

Step S260 preferably calibrates interference mapping by generatingcalibration references, which are then used to predict antenna fields(or a related metric, such as areas of super-threshold power).Calibration references may be pre-generated; for instance, calibrationreferences may be formed by a robot with an RFID tag traversing an area;the robot maps out the area while one or more constructive interferencepatterns are generated by antennas. The robot map may be, through timesynchronization, matched up to points of RFID tag activation, this dataset is then compared to predicted data. Transmission parameters may thenbe adjusted (e.g., by adjusting transmission variable inputs to aprediction engine until prediction matches reality, or by adjustingactual transmission variable inputs until the robot output matchespredictions). Pre-generated calibration references may be calculated inreal-time with data gathering (e.g., as the robot moves around) or at alater time.

The calibration references may additionally or alternatively begenerated in real-time (during operation of the method 200). In oneexample, RFID reference tags are placed in known locations. These may beused to generate calibration references at any time. This allows foreasy recalibration when environmental factors (e.g., positioning ofRF-signal-affecting objects, etc.) change.

These calibration references are preferably measured for a range oftransmission parameters. In particular, calibration referencespreferably contain enough information to accurately calculate antennafields for significantly varying transmission parameters (e.g.,transmission phase and power from each antenna). In some cases, this maynot mean actually measuring fields for wide ranges of all parameters.For instance, if the phase delay of signals is independent oftransmission power for a particular environment, and the antennatransmission characteristics are well-known, an accurate calibration maynot require many data points at different transmission powers.

Predicted fields are preferably generated by a modeling of theconstructive interference fields based on calibration data collected aspart of the method 200; additionally, the modeling may also be based onadditional data. In one example, antenna fields for a particular set oftransmission parameters not exactly sampled as part of calibration datamay be predicted by interpolating calibration data. In another example,calibration data is used to model the permittivity (vs. coordinates) ofa volume of interest, which are then used to predict antenna fields atany transmission power and phase.

Calibration may additionally or alternatively be performed with the aidof non-STSRM techniques. For example, persons in a particular area maycarry RFID tags that identify them. If the position of the persons canbe located with precision (e.g., by a camera, or by another method, suchas detecting wireless transmissions from their cellphone), the RFID tagsthey carry could be used to generate calibration data. Cameras or otherlocating methods may additionally or alternatively be used at any pointin the method 200 in order to refine or calibrate STSRM location data.

Step S260 may additionally include calibrating read probabilities bygenerating read probability references. Read probability references maybe pre-generated; for instance, read probability references may beformed by a robot with an RFID tag traversing an area; the robot mapsout the area while signals are output from one antenna (or serially frommultiple antennas). The robot map, through time synchronization, may bematched up to points of RFID tag activation; this data may then be usedto calculate read probabilities as a function of position in the area.As in the previous process, the transmission variables may be adjustedto match predictions to reality or vice versa. Read probabilitycalibration processes may be performed in real-time with data gathering(e.g., as the robot moves around) or at a later time (using previouslycorrected data).

The description of the method 200 above provides examples directed tolocating particular tags or of tags in a sparse environment; that is,scenarios where some amount of search is required to find a tag. Aperson skilled in the art will recognize that the method 200 is alsoapplicable to systems where a large number of tags are located in somearea, and the locations of many or all of those tags are of interest. Inexamples applying to such a situation, constructive interferencepatterns may not be generated to find a particular tag, but rather toprovide the locations of all tags within a certain area; the strategy tolocate all tags in an area (e.g., what patterns are generated and inwhat order) may be significantly different than a strategy to locate asingle tag.

The method 200 is preferably performed by the system 100 but mayadditionally or alternatively be performed by any suitable system.

The methods of the preferred embodiment and variations thereof can beembodied and/or implemented at least in part as a machine configured toreceive a computer-readable medium storing computer-readableinstructions. The instructions are preferably executed bycomputer-executable components preferably integrated with an RFID taglocating system. The computer-readable medium can be stored on anysuitable computer-readable media such as RAMs, ROMs, flash memory,EEPROMs, optical devices (CD or DVD), hard drives, floppy drives, or anysuitable device. The computer-executable component is preferably ageneral or application specific processor, but any suitable dedicatedhardware or hardware/firmware combination device can alternatively oradditionally execute the instructions.

The disclosed embodiments are susceptible to various modifications andalternative forms, and specific examples thereof have been shown by wayof example in the drawings and herein described in detail. It should beunderstood, however, that the disclosed embodiments are not meant to belimited to the particular forms or methods disclosed, but to thecontrary, the disclosed embodiments are to cover all modifications,equivalents, and alternatives.

What is claimed is:
 1. A method, comprising: transmitting a first signal from a first antenna towards a radio-frequency identification (RFID) tag; and transmitting a second signal from a second antenna towards the RFID tag, the second signal constructively interfering with the first signal within a constructive interference region containing the RFID tag, wherein the first signal has a first amplitude below an activation threshold of the RFID tag, the second signal has a second amplitude below the activation threshold of the RFID tag, and the first signal and the second signal constructively interfere to produce constructive interference having an amplitude above the activation threshold within the constructive interference region.
 2. The method of claim 1, further comprising: recharging the RFID tag via the constructive interference.
 3. The method of claim 1, further comprising: receiving a response from the RFID tag at the first antenna in response to the constructive interference.
 4. The method of claim 3, further comprising: estimating a location of the RFID tag based on a location of the constructive interference region and the response from the RFID tag.
 5. The method of claim 1, further comprising: altering a transmission parameter of the first signal so as to change the constructive interference region.
 6. The method of claim 5, further comprising: estimating a location of the RFID tag based on a change in the constructive interference region caused by altering the transmission parameter of the first signal.
 7. The method of claim 5, wherein altering the transmission parameter of the first signal comprises changing a relative phase of the first signal with respect to the second signal so as to move the constructive interference region with respect to the RFID tag.
 8. The method of claim 6, wherein altering the transmission parameter comprises selecting the relative phase so as to move the constructive interference region with respect to the RFID tag by less than a wavelength of the first signal.
 9. The method of claim 1, further comprising: acquiring an image of the constructive interference region; and associating an object and/or person in the image of the constructive interference region with the RFID tag.
 10. The method of claim 1, further comprising: transmitting a third signal from a third antenna towards the RFID tag, the third signal constructively interfering with the first signal and the second signal over at least a portion of the constructive interference region.
 11. The method of claim 10, further comprising: estimating a three-dimensional location of the RFID tag based on a location of the at least a portion of the constructive interference region.
 12. The method of claim 1, further comprising: generating the first signal based on historical data representing a location of the RFID tag.
 13. The method of claim 1, further comprising: receiving a first response signal, in response to the first signal and the second signal, from a reference RFID tag located at a first known location within the constructive interference region.
 14. The method of claim 13, further comprising: moving the reference RFID tag to a second known location; altering a transmission parameter of the first signal so as to change the constructive interference region to cover the second known location; and receiving a second response signal from the reference RFID tag.
 15. A system, comprising: a first antenna configured to transmit a first signal from a first antenna towards a radio-frequency identification (RFID) tag; and a second antenna configured to transmit a second signal towards the RFID tag such that the second signal constructively interferes with the first signal within a constructive interference region containing the RFID tag, wherein the first antenna is configured to transmit the first signal at a first amplitude below an activation threshold of the RFID tag, the second antenna is configured to transmit the second signal at a second amplitude below the activation threshold of the RFID tag, and the first signal and the second signal constructively interfere to produce constructive interference having an amplitude above the activation threshold over the constructive interference region.
 16. The system of claim 15, wherein the first antenna and the second antenna are configured to remotely recharge the RFID tag.
 17. The system of claim 15, wherein the first antenna is further configured to receive a response from the RFID tag.
 18. The system of claim 17, further comprising: a controller, operably coupled to the first antenna and the second antenna, to estimate a location of the RFID tag based on a location of the constructive interference region and the response from the RFID tag.
 19. The system of claim 18, wherein the controller is further configured to alter a transmission parameter of the first signal so as to change the constructive interference region.
 20. The system of claim 19, wherein the controller is further configured to estimate the location of the RFID tag based on a change in the constructive interference region caused by altering the transmission parameter of the first signal.
 21. The system of claim 18, further comprising: a phase shifter, operably coupled to the controller and the first antenna, to change a relative phase of the first signal with respect to the second signal so as to move the constructive interference region with respect to the RFID tag.
 22. The system of claim 21, wherein the phase shifter is configured to change the relative phase by an amount selected to move the constructive interference region with respect to the RFID tag by less than a wavelength of the first signal.
 23. The system of claim 18, further comprising: a camera configured to acquire an image of the constructive interference region, and wherein the processor is configured to associate an object and/or person in the image of the constructive interference region with the RFID tag.
 24. The system of claim 15, further comprising: a third antenna configured to transmit a third signal from towards the RFID tag, the third signal constructively interfering with the first signal and the second signal over at least a portion of the constructive interference region.
 25. The method of claim 24, further comprising: a controller, operably coupled to the first antenna, the second antenna, and the third antenna, to estimate a three-dimensional location of the RFID tag based on a location of the at least a portion of the constructive interference region.
 26. The system of claim 15, wherein the first signal is based on historical data representing a location of the RFID tag.
 27. The system of claim 15, further comprising: a reference RFID tag, located at a first known location within the constructive interference region, to transmit a first response signal to the first antenna in response to constructive interference of the first signal and the second signal.
 28. The system of claim 27, further comprising: a robot configured to move the reference RFID tag from the first known location to a second known location.
 29. A method, comprising: transmitting a first signal from a first antenna towards a receiver; and transmitting a second signal from a second antenna towards the receiver, the second signal constructively interfering with the first signal within a constructive interference region containing the receiver, wherein the first signal has a first amplitude below an activation threshold of the receiver, the second signal has a second amplitude below the activation threshold of the receiver, and the first signal and the second signal constructively interfere to produce constructive interference having an amplitude above the activation threshold within the constructive interference region.
 30. The method of claim 29, further comprising: recharging the receiver via the constructive interference.
 31. The method of claim 29, further comprising: receiving a response from the receiver at the first antenna in response to the constructive interference.
 32. The method of claim 31, further comprising: estimating a location of the receiver based on a location of the constructive interference region and the response from the receiver.
 33. The method of claim 29, further comprising: altering a transmission parameter of the first signal so as to change the constructive interference region.
 34. The method of claim 33, further comprising: estimating a location of the receiver based on a change in the constructive interference region caused by altering the transmission parameter of the first signal.
 35. The method of claim 33, wherein altering the transmission parameter of the first signal comprises changing a relative phase of the first signal with respect to the second signal so as to move the constructive interference region with respect to the receiver.
 36. The method of claim 34, wherein altering the transmission parameter comprises selecting the relative phase so as to move the constructive interference region with respect to the receiver by less than a wavelength of the first signal.
 37. The method of claim 29, further comprising: acquiring an image of the constructive interference region; and associating an object and/or person in the image of the constructive interference region with the receiver.
 38. The method of claim 29, further comprising: transmitting a third signal from a third antenna towards the receiver, the third signal constructively interfering with the first signal and the second signal over at least a portion of the constructive interference region.
 39. The method of claim 38, further comprising: estimating a three-dimensional location of the receiver based on a location of the at least a portion of the constructive interference region.
 40. The method of claim 29, further comprising: generating the first signal based on historical data representing a location of the receiver.
 41. The method of claim 29, further comprising: receiving a first response signal, in response to the first signal and the second signal, from a reference receiver located at a first known location within the constructive interference region.
 42. The method of claim 41, further comprising: moving the reference receiver to a second known location; altering a transmission parameter of the first signal so as to change the constructive interference region to cover the second known location; and receiving a second response signal from the reference receiver.
 43. A method, comprising: transmitting a first signal from a first antenna towards a radio-frequency identification (RFID) tag; transmitting a second signal from a second antenna towards the RFID tag, the second signal constructively interfering with the first signal within a constructive interference region containing the RFID tag; acquiring an image of the constructive interference region with a camera, the image of the constructive interference region showing an object and/or a person; and associating the object and/or the person in the image of the constructive interference region with the RFID tag.
 44. A method, comprising: transmitting a first signal from a first antenna towards a first known location; transmitting a second signal from a second antenna towards the known location, the second signal constructively interfering with the first signal at the first known location; receiving a first response signal, in response to the first signal and the second signal, from a reference RFID tag located at a first known location; moving the reference RFID tag to a second known location; altering a transmission parameter of the first signal so as to cause the first signal and the second signal to constructively interfere at the second known location; and receiving a second response signal from the reference RFID tag in response to the first signal and the second signal. 